PPC: Use r2 instead of r29 as global data pointer

R29 was an unlucky choice as with recent toolchains (gcc-4.2.x) gcc
will refuse to use load/store multiple insns; instead, it issues a
list of simple load/store instructions upon function entry and exit,
resulting in bigger code size, which in turn makes the build for a
few boards fail.

Use r2 instead.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/cpu/mpc5xx/config.mk b/cpu/mpc5xx/config.mk
index 64cd600..6d66c32 100644
--- a/cpu/mpc5xx/config.mk
+++ b/cpu/mpc5xx/config.mk
@@ -30,7 +30,7 @@
 
 PLATFORM_RELFLAGS += 	-fPIC -ffixed-r14 -meabi
 
-PLATFORM_CPPFLAGS +=	-DCONFIG_5xx -ffixed-r2 -ffixed-r29 -mpowerpc -msoft-float
+PLATFORM_CPPFLAGS +=	-DCONFIG_5xx -ffixed-r2 -mpowerpc -msoft-float
 
 # Use default linker script.  Board port can override in board/*/config.mk
 LDSCRIPT := $(SRCTREE)/cpu/mpc5xx/u-boot.lds